1. Field of the Invention
The present invention relates to a lithium secondary battery.
2. Description of the Related Art
With a view point of environmental protection and energy saving, hybrid cars that use in combination an engine and a motor together as a power source have been developed and put to actual products. Further, development of fuel cell type hybrid cars that use fuel cells instead of engines has been made vigorously for future use.
Secondary batteries capable of charging/discharging electric energy repetitively are technically essential as energy sources for hybrid cars.
Among them, a lithium secondary battery is a battery having a feature of a high energy-density, where operation voltage is high and high power is obtained easily. Such a lithium secondary battery is important more and more as the power source of the hybrid cars in the future.
For the lithium secondary battery which is used as the power source for the hybrid cars, one of its technical subjects is to suppress the increase of resistance during high temperature storage at 50° C. or higher.
To suppress the increase of resistance during storage at high temperature, a countermeasure of adding a compound such as vinylene carbonate to an electrolyte has been proposed.
A battery in which deterioration during storage at 60° C. can be suppressed by adding 2 wt % vinylene carbonate to an electrolyte comprising, for example, LiPF6, ethylene carbonate, and dimethyl carbonate has been proposed in Journal of The Electrochemical Society, 151 (10) A1659-A1669 (2004).